1. Technical Field
The present invention relates to a signal output circuit that has a signal output transistor.
2. Related Art
For example, there is a switching apparatus that is configured using a metal-oxide-semiconductor field-effect transistor (MOSFET) for outputting a control signal to a controlled-apparatus side in a programmable controller. In a switching apparatus such as that above, JP-H08-242157 discloses the following configuration to prevent the occurrence of a flash-on phenomenon when power is turned on.
This switching apparatus includes an N-channel MOSFET (hereinafter referred to as “FET”), a PNP transistor, and two resistor elements (first and second resistor elements). A gate of the FET is connected to the second resistor element of which one end is connected to a base of the PNP transistor and the other end is connected to an emitter of the PNP transistor. A collector of the PNP transistor is connected to a source of the FET. The first resistor element is connected between the base and the collector of the PNP transistor.
In the configuration above, when power is turned on, if a current flows from the drain of the FET to the gate side of the FET via a junction capacitance Cgd in the FET, the current flows to the two resistor elements and the PNP transistor is turned on. Therefore, flash-on is prevented as a result of the gate of the FET being held low. In addition, in this configuration, when the FET is turned on, a current for discharging the charged electric charge of a junction capacitance Cgs in the PET also flows to the two resistor elements. The PNP transistor is thereby turned on. Thus, the above-described discharging is promoted, off delay time can also be shortened.
In the switching apparatus such as that described above, there is a constant demand for size reduction. When the inventors of the present application confirmed operation after actually reducing the size of the FET included in the switching apparatus in JP-H08-242157, a problem occurred in that, as a result of decrease in the junction capacitance of the FET, while the effect of shortening the off delay time was unaffected, the flash on/off phenomenon could no longer be prevented.